Solid state lighting (SSL), e.g. LED lighting, is rapidly gaining popularity because of its energy credentials and superior lifetime compared to traditional lighting, e.g. incandescent lighting, fluorescent lighting and halogen lighting. Nevertheless, market penetration of such SSL devices is not without challenges. For example, purchase cost of SSL devices is still higher than that of equivalent traditional light sources, even though the effective cost of such SSL devices is markedly lower due to their much longer lifetime.
Another serious challenge is to provide a luminaire including SSL elements that offers the same visual experience as such traditional light sources. This is a far from trivial challenge, given that such luminaires typically comprise a plurality of SSL elements that act as point sources, which may lead to pixilation and glare in the luminous output of the SSL-based luminaire.
For this reason, some luminaire may include additional optical elements, e.g. diffusers, to diffuse the luminous output of the luminaire, thereby reducing pixilation and glare effects. One particular class of such diffusers are light guides, which are optical bodies comprising opposing total internal reflection surfaces, where light from an SSL element such as a LED is typically coupled into the light guide through an edge surface extending between the opposing total internal reflection surfaces. Light is coupled out of the light guide using outcoupling structures, e.g. on one of the surfaces, which disrupt the total internal reflection and allow for light to escape the light guide. Therefore, by arranging the outcoupling structures in a particular pattern, the luminous output distribution of the light guide may be controlled to a certain degree. An example of a light guide is disclosed in U.S. Pat. No. 8,033,706 B1.
In certain application domains, e.g. outdoor lighting, it may be aesthetically desirable to produce an asymmetric beam having good diffusiveness characteristics, i.e. having a uniformly lit light exit surface. It is not trivial to meet these requirements with light guides.
In some cases, a light output is desired which does not show pixilation, but which is also asymmetrical.
One solution proposed by the applicant is to provide a light guide for mounting horizontally and having an arrangement of light sources around at least part of the edge. By selecting which light sources are turned on, an asymmetric light output may be created. This enables an asymmetric beam to be formed having good uniformity, i.e. a beam formed with substantially the entire light exit surface, and with directional control.
However, this design requires the light sources to be mounted against the edge, requiring a flexible carrier circuit board or else individual mounting of the light sources.